A windmill as early disclosed by the same applicant of this invention of U.S. Pat. No. 4,649,284 is shown in FIG. 10 in which the fan blades 3 when rotated to the left side as shown in the figure are expected to be feathered to reduce the wind resistance on the left side. However, when the wind is very strong to exert a great moment of inertia of the blades 3 rotating around a longitudinal axis of the transmission shaft 2, the blades 3 once rotated towards the left side may not be feathered due to the great moment of inertia or the centrifugal force of the rotating blades as shown in dotted lines of FIG. 10, thereby counteracting the wind driving force acting on the right side blades and greatly reducing an output energy of the windmill.
The present invention has found the shortcoming of my early patented windmill and invented the present windmill for automatically restoring the fan blades for always facing the wind force when rotated from windward towards leeward.